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8 thoughts on “Biofuel from carbon capture”

I’m having a little problem here with the laws of thermodynamics: how can we capture carbon from an energy-releasing carbon reaction without spending more energy than was released by the reaction in the first place? Isn’t that like perpetual motion?

When a fuel– say methane– is burned in order to utilize the energy, energy is released by the oxidation process. My distant recollection from chemistry classes long ago is: CH4 + 2O2 -> CO2 + 2H2O + energy. In order to sequester the carbon from the CO2, energy must be added back. The laws of thermodynamics (strongly) suggest that it would take as much energy to sequester the carbon as was released when the carbon oxidized– assuming 100% efficiency. If 100% efficiency is not possible, then it would take MORE energy to sequester the carbon than was released through the oxidation process. Unless I’m missing something.

It is: once the fuel has been burned, you can’t convert it back into fuel without adding back the energy that was released during combustion. For example, 2H2 + O2 -> 2H2O + energy. To convert the water back into hydrogen, you’ve got to add energy. In this case, H20 is easily converted to H2 by adding electricity, and therefore excess electricity could foreseeably be used to create Hydrogen– or, in other words, hydrogen is a convenient way to store excess energy.

Even in the best case, 100% eficiency, it would take all of the energy produced by birning H2 to convert the byproduct (water) back into H2. But the process is far less than 100% efficient. Converting energy to H2, stporing it (on-site, no treansportation), and burning it later is about 50% efficient with currently available technologyu. In other words, it trakes twioce as much energy to convert water to H2 as is released when you burn the H2.

Carbon on the other hand is less simple– it’s not just a matter of electrolysis– and you’re not talking about using spare energy, you’re talking about converting every carbon molecule burned. As far as I can see, that’s just a newer version of the perpetual motion machine– impossible under the laws of physics.

DJ: The algae do photosynthesis, that’s how it captures carbon. The energy input required is from sunlight. This is a solar energy scheme.

My problem with this scheme is that it is not carbon neutral; it’s more like carbon-once-recycle. The coal’s carbon goes into the air after a phase of being in biodiesel. Truly carbon-neutral algae biodiesel requires taking CO2 from the air. So either air as feedstock (but that’s less efficient) or concentrating CO2 from the air (that uses energy). Or perhaps from burning non-fossil carbon (trash, etc.).